<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div>

        <div>商品数量 <input type="text" id="goodsnum"/></div>
        <div>商品价格 <input type="text" id="goodsprice"/></div>
        <div>
           小计 <span id="total"></span>
        </div>
    </div>
    
</body>
<script>
        let  goodsnumInput = document.getElementById("goodsnum");
        let  goodspriceInput = document.getElementById("goodsprice");
        let totalSpan = document.getElementById("total");


        goodsnumInput.oninput  =function(){
            totalSpan.innerHTML = goodsnumInput.value  * goodspriceInput.value;
        }
        goodspriceInput.oninput = function(){
            totalSpan.innerHTML = goodspriceInput.value * goodsnumInput.value;
        }


</script>
</html>